Processes for testing for leakage currents in lambda probes are known. In general, lambda probes have two electrodes with a solid electrolyte between them. Furthermore, a heating element is provided, which is electrically insulated from both electrodes by means of an insulating layer. If this insulating layer has one or more electric leakage points, a leakage current is formed between the heating element and one of the electrodes; as oxygen concentration is measured, the current is superimposed on the measuring current in the lambda probe and distorts the measurement results.
To measure this leakage current, a voltage is usually applied to the heating element, and the current flowing through the electrode referred to as the external electrode is measured. For this purpose, the external electrode is connected to ground via an ammeter. If the insulation between the heating element and this external electrode has an electric leakage point, the ammeter will show a value after a voltage is applied to the heating element which, if it exceeds a limit value, indicates a leakage point. This leakage current, established after a transient process, forms the quasi-stationary leakage current hereinafter referred to as leakage current.
The disadvantage of this method is that O.sup.2- ions accumulate to form a space charge at the leakage point in the insulation layer. An electric field, which is of opposite polarity with respect to the field between the heating element and the external electrode, is formed between the space charge and the heating element, thereby reducing the ion stream flowing to the external electrode. Therefore smaller leakage points in the insulation cannot be detected.
Another disadvantage of this method consists of the fact that the external electrode and one terminal of the heating element are at a common potential. Therefore leakage points in the proximity of this terminal of the heating element cannot be detected due to the lack of difference in potential.